package org.vesselonline.mail;

import javax.swing.SwingUtilities;

/**
 * This is the 3rd version of SwingWorker (also known as SwingWorker 3), an
 * abstract class that you subclass to perform GUI-related work in a dedicated
 * thread. For instructions on and examples of using this class, see:
 * http://java.sun.com/docs/books/tutorial/uiswing/misc/threads.html Note that
 * the API changed slightly in the 3rd version: You must now invoke start() on
 * the SwingWorker after creating it.
 */
public abstract class SwingWorker {
  private Object value;  // see getValue(), setValue()

  /**
   * Class to maintain reference to current worker thread under separate
   * synchronization control.
   */
  private static class ThreadVar {
    private Thread thread;

    ThreadVar(Thread t) { thread = t; }

    synchronized Thread get() { return thread; }
    synchronized void clear() { thread = null; }
  }

  private ThreadVar threadVar;

  /**
   * Get the value produced by the worker thread, or null if it hasn't been
   * constructed yet.
   */
  protected synchronized Object getValue() { return value; }

  /** Set the value produced by worker thread. */
  private synchronized void setValue(Object x) { value = x; }

  /** Compute the value to be returned by the <code>get</code> method. */
  public abstract Object construct();

  /**
   * Called on the event dispatching thread (not on the worker thread) after the
   * <code>construct</code> method has returned.
   */
  public void finished() { }

  /**
   * A new method that interrupts the worker thread. Call this method to force
   * the worker to stop what it's doing.
   */
  public void interrupt() {
    Thread t = threadVar.get();
    if (t != null) t.interrupt();
    threadVar.clear();
  }

  /**
   * Return the value created by the <code>construct</code> method. Returns
   * null if either the constructing thread or the current thread was
   * interrupted before a value was produced.
   * @return the value created by the <code>construct</code> method
   */
  public Object get() {
    while (true) {
      Thread t = threadVar.get();
      if (t == null) return getValue();

      try {
        t.join();
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt();  // propagate
        return null;
      }
    }
  }

  /** Start a thread that will call the <code>construct</code> method and then exit. */
  public SwingWorker() {
    final Runnable doFinished = new Runnable() {
      public void run() { finished(); }
    };

    Runnable doConstruct = new Runnable() {
      public void run() {
        try {
          setValue(construct());
        } finally {
          threadVar.clear();
        }

        SwingUtilities.invokeLater(doFinished);
      }
    };

    Thread t = new Thread(doConstruct);
    threadVar = new ThreadVar(t);
  }

  /** Start the worker thread. */
  public void start() {
    Thread t = threadVar.get();
    if (t != null) t.start();
  }
}
